The present invention generally relates to facsimile machines, and more particularly to a facsimile machine having an error correction mode.
In the present specification, a facsimile machine which sends an image information will be referred to as a source facsimile machine, and a facsimile machine which receives the image information from the source facsimile machine will be referred to as a destination facsimile machine.
When transmitting an image information from a source facsimile machine, the image information cannot be accurately reproduced on a destination facsimile machine if a transmission error exists in the received image information such as a case where the state of the line is poor due to a line noise or the like. Hence, an error correction mode (ECM) has been recommended by the CCITT as a standard system for eliminating such a problem. According to this CCITT recommendation, an image information which is coded and compressed is divided into a frame for every 256 bytes (octets) or 64 bytes from a beginning of the image information. As shown in FIG. 1A, the image information amounting to one frame is transmitted in frames FLM in conformance with a high level data link control (HDLC) procedure.
The frame FLM is made up of a (start) flag sequence F having a predetermined bit pattern, an address field A having a predetermined bit pattern (global address), a control field C having a bit pattern peculiar to the facsimile machine, an information field I, a frame check sequence FCS for making an error detection, and a (end) flag sequence F. The information field I includes a facsimile control field FCF in which a facsimile transmission procedure signal is arranged, and a facsimile information field FIF in which various information added to the facsimile transmission procedure signal is arranged.
A facsimile coding data FCD of the facsimile transmission procedure signal is arranged in the facsimile control field FCF. A frame number FNo which indicates the order (sequence) of the frames and a coded frame data FDc having a frame size FSZ are arranged in the facsimile information field FIF. Because the frame number FNo is an 8-bit binary number and only 256 consecutive numbers "0" to "255" can be described thereby, 256 consecutive frames are set to one block and the destination facsimile machine makes a retransmission request in blocks when a transmission error is detected. When the image information amounting to one page cannot be transmitted in one block, the image information related to a remaining portion of the page is transmitted in a next block.
When making the retransmission request, the destination facsimile machine sends a frame of a facsimile transmission procedure signal to the source facsimile machine. The facsimile transmission procedure signal is transmitted and received in a state including the required parameters with a frame format similar to that of a partial page request (PPR) signal, and will hereinafter simply be referred to as the PPR signal.
The PPR signal is arranged in the facsimile control field FCF of the information field I with a bit pattern which indicates that the signal is the PPR signal. On the other hand, a 256-bit error map data EMp is arranged in the facsimile information field FIF of the information field I. The error map data EMp has a data "0" or "1" for each of the frame of the received block in the order of the frames. The error map data EMp has the data "0" when no transmission error exists in the frame data of the frame and has the data "1" when a transmission error exists in the frame data of the frame. Hence, when the source facsimile machine receives the PPR signal, the source facsimile machine retransmits to the destination facsimile machine only the frame data of the frame which has the data "1" in the error map data EMp.
By repeating the retransmission request until the transmission error in all of the frames is eliminated, it becomes possible to record on a recording sheet an error-free image on the destination facsimile machine.
When receiving the image information in the ECM, the destination facsimile machine is provided with a pair of receiver buffers (memories) BF1 and BF2 which are respectively shown in FIGS. 2A and 2B. For example, the receiver buffers BF1 and BF2 respectively have a memory capacity of 64k (k=1024 bits) which is sufficient to store image information amounting to one block. The receiver buffer BF1 stores the image information of a first page. Then, the receiver buffer BF2 stores the image information of a second page, and at the same time, the image information of the first page stored in the receiver buffer BF1 is read out and recorded on the recording sheet. In other words, the recording of the image information which is already received and the reception of the image information related to a next page are carried out in parallel so as to minimize a time required to make the image information transmission from the source facsimile machine.
However, the following problems occur in the conventional facsimile machine having the ECM. That is, the information quantity of the image information related to the second page may be extremely small, and the reception of the image information related to the second page may end before the recording of the image information related to the first page ends. But in this case, it is impossible to start receiving the image information related to a third page because a memory space for storing the image information related to the third page becomes available only after the recording of the image information related to the first page ends. As a result, there is a problem in that the reception of the image information related to the third page must wait until the recording of the image information related to the first page ends. This means that the time required to transmit the image information cannot be reduced even when the information quantity of the image information related to a certain page is small if the reading out of the stored image information is not ended from the reception buffer to which the image information related to the certain page is to be stored.